(1) Field of the Invention
This invention generally relates to antennas and more specifically to quadrifilar antennas.
(2) Description of the Prior Art
Numerous communication networks utilize omnidirectional antenna systems to establish communications between various stations in the network. In some networks one or more stations may be mobile while others may be fixed land-based or satellite stations. Omnidirectional antenna systems are preferred in such applications because alternative highly directional antenna systems become difficult to apply, particularly at a mobile station that may communicate with both fixed land-based and satellite stations. In such applications it is desirable to provide an omnidirectional antenna system that is compact yet characterized by a wide bandwidth and a good front-to-back ratio with either horizontal or vertical polarization.
Some prior art omnidirectional antenna systems use an end fed quadrifilar helix antenna for satellite communication and a co-mounted dipole antenna for land based communications. However, each antenna has a limited bandwidth. Collectively their performance can be dependent upon antenna position relative to a ground plane. The dipole antenna has no front-to-back ratio and thus its performance can be severely degraded by heavy reflections when the antenna is mounted on a ship, particularly over low elevation angles. These co-mounted antennas also have spatial requirements that can limit their use in confined areas aboard ships or similar mobile stations.
The following patents disclose helical antennas that exhibit some, but not all, the previously described desirable characteristics:
U.S. Pat. No. 4,295,144 (1981) Matta et al.
U.S. Pat. No. 5,170,176 (1992) Yasunaga et al.
U.S. Pat. No. 5,198,831 (1993) Burrell et al.
U.S. Pat. No. 5,255,005 (1993) Terret et al.
U.S. Pat. No. 5,343,173 (1994) Balodis et al.
U.S. Pat. No. 5,635,945 (1997) McConnell
U.S. Pat. No. 5,793,173 (1998) Standke et al.
U.S. Pat. No. 4,295,144 to Matta et al. discloses a feed system for a helical CP antenna that features folded belt or phasing lines to reduce space and icing and wind loading problems. If two belt lines are used, they can be placed diametrically opposite each other to reduce mutual coupling.
In U.S. Pat. No. 5,170,176 (1992) to Yasunaga et al. a quadrifilar helix antenna includes four helix conductors wound around an axis in the same winding direction. Each helix conductor has a linear conductor which is parallel to its axis at either end or both ends of the helix conductor. The purpose of this structure is to reduce the effect of multipath fading due to sea-surface reflection in mobile satellite communications. Although this patent discloses an antenna that provides good front-to-back ratio, the transmission pattern from the antenna is also characterized by essentially forming two major lobes about 60.degree. from the forward direction so it is not truly omni-directional over a hemisphere.
U.S. Pat. No. 5,198,831 to Burrell et al. discloses a navigation unit for receiving navigation signals from a source, such as global positioning satellites. A directly mounted helical antenna includes antenna elements composed of a thin film of conductive material printed on a flexible dielectric substrate rolled into a tubular configuration.
In U.S. Pat. No. 5,255,005 to Terret et al., an antenna structure for L band communications has a quasi-hemispherical radiation pattern and is capable of having a relatively wide passband, so that it is possible to define two neighboring transmission sub-bands therein or, again, a single wide transmission band. The antenna is of the type comprising a quadrifilar helix formed by two bifilar helices positioned orthogonally and excited in phase quadrature, and including at least one second quadrifilar helix that is coaxial and electromagnetically coupled with said first quadrifilar helix.
U.S. Pat. No. 5,343,173 to Balodis et al. discloses a method of and apparatus for transmitting or receiving circularly polarized signals. The technique employs a phase shifting network for connection between an antenna and a radio transmitter or receiver to produce a phase shift when transmitting or to eliminate a phase shift when receiving. In one preferred embodiment, a dielectric substrate has a phase shifting network or printed circuit lines defining signal transmission paths between a radio connection terminal and a plurality of antenna element connection terminals for coupling a multi-element antenna and a radio. Each transmission path is phase shifted relative to an adjacent path by a predetermined amount by each path having progressively equally different electrical length to provide equal phase shift of a radio frequency signal progressively through the transmission paths. Adjacent path pairs are progressively joined at combiner nodes of equal power division by shunt connection line segments so that the power at each antenna connection terminal is equal to the power at the radio connection terminal divided by the number (typically four) of antenna terminals.
U.S. Pat. No. 5,635,945 (1997) to McConnell et al. discloses a quadrifilar helix antenna with four conductive elements arranged to define two separate helically twisted loops, one differing slightly in electrical length from the other. The two separate helically twisted loops are connected to each other in a way as to provide impedance matching, electrical phasing, coupling and power distribution for the antenna. The antenna is fed at a tap point on one of the conductive elements determined by an impedance matching network which connects the antenna to a transmission line. This patent utilizes microstrip techniques to feed and match through a partly balanced transmission line. As a result the resultant bandwidth is narrow.
U.S. Pat. No. 5,793,338 to Standke et al. discloses a quadrifilar antenna comprising four radiators which, in the preferred embodiment, are etched onto a radiator portion of a microstrip substrate. The microstrip substrate is formed into a cylindrical shape such that the radiators are helically wound. A feed network etched onto the microstrip substrate feed network provides 0.degree., 90.degree., 180.degree. and 270.degree. phase signals to the antenna radiators. The feed network utilizes a combination of one or more branch line couplers and one or more power dividers to accept an input signal from a transmitter and to provide therefrom the 0.degree., 90.degree., 180.degree. and 270.degree. signals needed to drive the antenna.
There exists a family of quadrifilar helixes that are broadband impedance wise above a certain "cut-in" frequency, and thus are useful for wideband satellite communications including Demand Assigned Multiple Access (DAMA) UHF functions in the range of 240 to 320 MHz and for other satelite communications functions in the range of 320 to 410 MHz. Typically these antennas have (1) a pitch angle of the elements on the helix cylindrical surface from 50 down to roughly 20 degrees, (2) elements that are at least roughly 3/4 wavelengths long, and (3) a "cut-in" frequency roughly corresponing to a frequency at which a wavelength is twice the length of one turn of the antenna element. This dependence changes with pitch angle. Above the "cut-in" frequency, the helix has an approximataely flat VSWR around 2:1 or less (about the Z.sub.o value of the antenna). Thus the antenna is broadband impedance-wise above the cut-in frequency. The previous three dimensions translate into a helix diameter of 0.1 to 0.2 wavelengths at the cut-in frequency.
For pitch angles of approximately 30 to 50.degree., such antennas provide good cardioid shaped patterns for satellite communications. Good circular polarization exists down to the horizon since the antenna is greater than 1.5 wavelengths long (2 elements constitute one array of the dual array, quadrifilar antenna) and is at least one turn. At the cut-in frequency, lower angled helixes have sharper patterns. As frequency increases, patterns start to flatten overhead and spread out near the horizon. For a given satellite band to be covered, a tradeoff can be chosen on how sharp the pattern is allowed to be at the bottom of the band and how much it can be spread out by the time the top of the band is reached. This tradeoff is made by choosing where the band should start relative to the cut-in frequency and the pitch angle.
For optimum front-to-back ratio performance, the bottom of the band should start at the cut-in frequency. This is because, for a given element thickness, backside radiation increases with frequency (the front-to-back ratio decreases with frequency). This decrease of front-to-back ratio with frequency limits the antenna immunity to multipath nulling effects.
My above-identified pending United States Letters Patent (Ser. No. 09/356,803) discloses an antenna having four constant-width antenna elements wrapped about the periphery of a cylindrical support. This construction provides a broadband antenna with a bandwidth of 240 MHz to at least 400 MHz and with an input impedance in a normal range, e.g., 100 ohms. This antenna also exhibits a good front-to-back ratio in both open-ended and shorted configurations. In this antenna, each antenna element has a width corresponding to about 95% of the available width for that element. However, it has been found that such wide elements increase backside radiation and therefor degrade an idealized front-to-back ratio. In addition, the weight of the antenna elements at such widths approaches maximum limits in many applications, particularly satellite applications. What is needed is a wideband antenna that provides good cardioid patterns with circular polarization, a good front-to-back ratio and a construction that minimizes the weight of the antenna elements.